At present, a DC buck technique is mainly implemented with a linear regulator or a buck switching converter. The linear regulator adopts one power transistor (such as a bipolar junction transistor or a field effect transistor) and the power transistor is operated in the linear mode and the power transistor is equivalent to a variable resistor coupled to an output load in series at the moment. It is obvious that the output current will flow through the power transistor. Therefore, when the difference between the input voltage and the output voltage of the power convertor increases in the linear regulator, the linear regulator will cause great power consumption.
The buck switching converter also has a power transistor, which can be operated as a switch and only operated in the saturation mode and the cutoff mode. As to the buck switching converter, the relation between the input voltage and the output voltage depends on the duly cycle of the power transistor. Therefore, when the input voltage and the output voltage of the power convertor in the buck switching converter are close to each other, the duty cycle of the power transistor must extremely approach one theoretically. However, the manufactory technique for electric elements at present is hard to promote the transistor with such high duty cycle.
For an uninterrupted power supply or a portable power bank implemented with the DC buck technique, according to the different purposes, they may be designed to include a set of cells rather than one cell. Taking a lithium-ion battery with four lithium-ion cells in series for example, its float range of working voltage would be four times the float range of one lithium-ion cell. As shown in FIG. 1, the float range for one cell is 4.2V to 2.8V. Thus the float range for a lithium-ion battery would be 16.8V to 11.2V. Using more cells in series is able to adapt to the condition for higher output voltage, however that causes the increase in the float range of working voltage. It is a dilemma when designing a power convertor.